PARIS, March 16 (Reuters) - The fraud investigation into French presidential candidate Francois Fillon has been widened to include luxury suits he received as gifts, a judicial source said on Thursday.

The former prime minister had already been placed under formal investigation on Tuesday on suspicion of misusing public funds linked to salaries he paid his wife and children.

The expanded investigation came after the financial prosecutor asked judges to include suspicion of corruption to the list of possible charges, the source said, confirming an earlier report by Le Parisien newspaper.

Fillon, 63, denies wrongdoing and has refused to abandon his bid for the presidency even though the scandals have hammered his popularity ratings and only one in five voters now say they find him 'credible'.

Opinion polls indicate suggest he will be eliminated in the first round of voting on April 23.

The Journal du Dimanche newspaper on Sunday reported that Fillon had received close to 50,000 euros worth of suits and clothing since 2012.

Fillon acknowledged a friend had given him two suits in February this year, adding: "and so what". He gave no further details on the provider of the gift and said the story was proof of a campaign to wreck his presidential bid.

Once the frontrunner, Fillon is battling to rally supporters and keep his centre-right alliance on his side.

"My battle is about ideas. It has always been so. I have never howled with the wolves", Fillon said on Thursday night at a meeting in Caen. "I have never rummaged through the garbage of my political rivals."